Graylog2 not starting

I had a issue recently where I had to hard reset a server, afterwards Graylog2 would not start up.
Upon investigation it was found that mongoDB did not start up, this was due to a .lock file still present (not cleanly removed on reset)

to remove the lock:

sudo rm /var/lib/mongodb/mongod.lock

and then start mongodb with

sudo service mongodb start

If it allocates a process id (pid) to it you know it is up and running, can be further confirmed with 

sudo service mongodb status